1. Field of the Invention
The present invention concerns a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known modality for acquiring images of the inside of a body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields, which are generated by a gradient coil system, are superimposed on a static homogenous basic magnetic field that is generated by a basic field magnet. The magnetic resonance device also has a radio-frequency system that radiates radio-frequency signals into the examination subject to excite magnetic resonance signals, and that acquires the generated magnetic resonance signals, on the basis of which magnetic resonance images are created.
To generate the gradient field, appropriate currents are adjusted (set) in the gradient coil. The amplitudes of the required currents amount to more than 100 A. The current rise and fall rates amount to more than 100 kA/s. For power supply, each gradient coil is connected to a gradient amplifier. Since the gradient coil system normally is surrounded by electrically conductive structures, eddy currents are induced by the switched gradient fields. Examples of such conductive structures are the vacuum vessel and/or the cryoshield of a superconducting magnet used as the basic field magnet. The fields generated as a consequence of the eddy currents are undesirable because, without counteracting measures, they weaken the gradient fields and distort their time curves, which leads to an impairment of the quality of magnetic resonance images.
The impairment of a gradient field as a result of the eddy current fields can be compensated up to a certain degree by a corresponding predistortion of a quantity controlling the gradient field. To compensate, the controlling quantity is thereby filtered such that eddy current fields ensuing given non-predistorted operation of the gradient coil are cancelled by the predistortion. A filter network can be used for filtering having parameters determined by the time constants and coefficients that can be determined, for example, with a method described in German OS 198 59 501.
Furthermore, the eddy currents induced on a predeterminable enveloping surface (that, for example, proceeds through an inner cylinder jacket of an 80-K cryoshield of the superconducting basic field magnet) by the gradient coils being fed current can be reduced by the use of an actively-shielded gradient coil system. For this purpose a secondary coil associated with the gradient coil, normally having a lower number of windings than the gradient coil, is connected with the gradient coil such that the same current that flows through the gradient coil flows through the secondary coil, but in the opposite direction. The secondary coil thereby has a weakening effect on the gradient field in the imaging volume. A gradient coil with an associated secondary coil to reduce a gradient field on a predeterminable enveloping surface is specified in British Application 2 180 943, for example.
A magnetic resonance apparatus is described in German OS 34 11 222 that has three gradient coils to generate gradient fields and at least one further coil arrangement, operable independently of the gradient coils, to generate a magnetic field in the direction of the basic magnetic field. The further coil arrangement is fashioned such that the magnetic field, non-linearly changes spatially, and such that a superimposition of the magnetic field with gradient fields yields a defined, temporal, spatial change of a magnetic flux density. The further coil arrangement is fashioned in an embodiment such that the magnetic field exhibits a spatial curve that corresponds to a spherical function of the second or third order. In particular, unwanted eddy current effects caused by the gradient fields can be corrected with the further coil arrangement.
A method is described in German OS 101 09 543 for operating a magnetic resonance apparatus with a gradient coil system having at least one gradient coil to generate a gradient field in an imaging volume and at least one shield coil, controllable independently of the gradient coil, to generate a shielding field with which the gradient field can be neutralized. In this method, the suitably fashioned shield coil can be operated independently of the gradient coil to compensate eddy current fields of the first and higher order in the sense of a series expansion of a spherical function. The shield coil alternatively can be operated as an active shielding coil and is therefore arranged between the gradient coil and the eddy current field source, and must be disposed, as is the gradient coil, a distance from the imaging volume. In an embodiment, the shield coil is connected in series with parts of the gradient coil which, in the operation mode “eddy current field compensation,” do not contribute to the gradient field generation in the imaging volume.
A magnetic resonance apparatus with a gradient coil system is known from German OS 101 56 770 in which an electrically-conductive structure is arranged and fashioned such that a magnetic field (caused by a gradient field due to induction effects) of the structure is similar (in the structure sense) to the gradient field, at least within an imaging volume of the magnetic resonance apparatus. In an embodiment, at least one part of the structure is barrel-shaped, as a component of the basic field magnet. Among other things, this allows the gradient coil system to be designed without secondary coils, since the undesirable consequences of the switched gradient fields due to the similarity of the magnetic field caused by the structure can be almost completely controlled by a predistortion. The complete control is based on the assumption that all eddy currents decay with the same time constant.
A switchable gradient coil based on saddle coils that can be operated as main or supplementary coils is known from German PS 199 17 058, wherein the supplementary coils exhibiting a correspondingly decreased covering angle are arranged for one gradient axis in the openings between coils that are arranged diametrically displaced in a radial plane for the other gradient axis. The supplementary coils lie, for example, either on an inner or outer radial position. The combination of various supplementary coils for various axes in the radial position enables a small-volume assembly of gradient coils. For example, the active shielding of gradient coils, with which eddy currents are prevented outside of the gradient coils, is mentioned in the prior art discussed in that document. The subject of eddy currents is not considered further, thus for example a conductive structure is not discussed in connection with the device disclosed in that document, in particular in connection with the main and intermediate coils. There is no description of how the main and supplementary coils should be connected to achieve the various desired performance characteristics (for example linearity, linearity volumes, shielding, . . . ), or of how the main and supplementary coils functionally interact, except for static and dynamic connections.